This invention relates to a valve tappet for an internal combustion engine and more particularly to an improved shoe concept that permits greater engine performance through more radical cam design and minimizes wear on the shoe, cam lobe and associated engine components.
As is well known in cam design for internal combustion engines, it is desirable to provide rapid valve lift which has the effect of maximizing the breathing capacity of the engine while permitting shorter duration than is possible with engines having slower valve lift rates. It has normally been the practice to utilize roller followers to permit cam designs which give such rapid lift rates. However, if the diameter of the roller which engages the cam surface is too small, serious limitations in cam design result. That is, the cam profile for such small radius rollers causes concave or reverse flanks which increase the likelihood of jamming at both the cam and roller engaging surface and unduly large cocking loads on the tappet. If standard acceleration rates are used, the cam lobes for such engines must be ground by means of extremely small diameter grinding wheels which has the effect of increasing the cost of the camshaft. Roller tappets in general also have the disadvantage of "edge riding." This results from the extreme difficulty in maintaining complete perpendicularity between the cam lobe surface and the roller surface with the result that the roller follower has a tendency to contact only the edge of the cam lobe resulting in very high wear on the lobe.
When the tappet is used with an engine having its camshaft in the block, the roller follower must be small enough so as to pass through the bore in which the tappet is supported. If this is not true, assembly and servicing of the engine becomes very difficult and major changes must be made to the engine block design to permit other forms of assembly.
For the foregoing reasons, it has been the common practice to provide tappets having substantially flat lower faces which contact the cam lobes.
Such flat face tappets do lend themselves to rapid valve opening and closure but have limitations as to the maximum velocity of valve movement which may be accomplished due to tappet diameter and nose radius development. Another problem inherent with cam and follower design for the valve train of an internal combustion engine is the maintenance of low unit pressure loads on the engaging surfaces. With conventional flat face tappets, which actually are tappets having a large spherical radius rather than being flat (about 60 inches radius), the contact between the cam and follower is at best line contact. The high unit loading obviously results in considerable wear and dictates low valve spring loads through more conservative valve opening and closure rates.
The difficulty in maintaining perpendicularity on the cam mating surfaces also gives rise to a situation known as cam walk. That is, misalignment or uneven contact between the cam follower and cam lobe surface exerts axial forces on the camshaft. With flat type follower it has been the practice to use a small degree of inclination so as to reduce such cam walk and to induce an axial force on the camshaft in the desired direction. This eliminates or reduces the necessity for providing thrust bearings for the camshaft. With flat type followers, this practice further aggravates the wear situation since it further minimizes the area of contact between cam and follower. It has been next to impossible to provide such inclined cam lobe surface with roller followers due to the edge riding problem aforedescribed.
It is therefore a principal object of this invention to provide an improved valve tappet for an internal combustion engine that minimizes wear on the components associated with the valve train.
It is another object of this invention to provide an improved valve tappet which minimizes the likelihood of binding of the tappet in its supporting bore and which also reduces the tendency for cam walk to occur.
It is still a further object of this invention to provide an improved valve tappet which improves contact area and permits more rapid valve opening and closing while still being capable of use in an overhead valve engine in which the camshaft is journaled in the engine block.
It is yet another object of the invention to provide an improved valve tappet that permits a wider latitude of valve timing and duration with a given camshaft and which facilitates changes in timing and duration to be made without necessitating replacement of the camshaft.